Articulated steering has been provided on work machines for many years. Such steering systems typically include a pair of fluid operated jacks connected between a pair of pivotally connected machine sections of the work machine. The jacks are extensibly movable to relatively pivot the machine sections and effect steering of the machine as it traverses the underlying terrain. The jacks are normally positioned on opposite sides of the pivot and a longitudinal machine axis so that extension of one jack and retraction of the other causes the articulated steering.
It has been known to provide multiple articulated steered machine sections on certain machines. For example, a forwarder used in collecting and carrying logs in forestry applications often has three consecutively pivotally connected machine sections. An example of such a machine is shown in U.S. Pat. No. 5,366,337, to Peter Eriksson, dated Nov. 22, 1994. In such machines, the first and second pairs of fluid operated steering jacks are separately controlled. Since there is no coordination between the steering of the first and second relatively steered machine sections and the second and third relatively steered machine sections, the steering radius of the pivotally connected first and second sections of the machine may differ from the steering radius of the pivotally connected second and third sections of the machine. This difference may, in extreme situations, cause excessive tire wear, increased fuel consumption, and reduced accuracy of machine controllability.
In some machine applications, the second pair of jacks has been eliminated, and the third section is allowed to trail the second section. This reduces the potential for gross forced steering misalignment but eliminates the ability to steer the third machine section, an advantage in rough terrain applications. In free trailing applications, the third section may wander and deviate from the optimum trailing location. In forwarder machine applications, improper positioning of the third section increases the amount of time to load logs. Also, such free following is detrimental to overall machine stability.
It is recognized that there are situations where independent articulated steering of the pivotally connected machine sections, free trailing of the pivotally connected machine sections, and optimized path controlled steering of the following machine section(s) are each desirable. At present, no steering system available is capable of providing controlled path articulated steering for the trailing section, or a combination of the above.
The present invention is directed to overcoming one or more of the problems set forth above.